Additive manufacturing techniques and processes generally involve the buildup of one or more materials to make a net or near net shape (NNS) object, in contrast to subtractive manufacturing methods. Though “additive manufacturing” is an industry standard term (ASTM F2792), additive manufacturing encompasses various manufacturing and prototyping techniques known under a variety of names, including freeform fabrication, 3D printing, rapid prototyping/tooling, etc. Additive manufacturing techniques are capable of fabricating complex components from a wide variety of materials. Generally, a freestanding object can be fabricated from a computer-aided design (CAD) model.
A particular type of additive manufacturing is more commonly known as 3D printing. One such process commonly referred to as Fused Deposition Modeling (FDM) comprises a process of melting a very thin layer of a flowable material (e.g., a thermoplastic material), and applying this material in layers to produce a final part. This is commonly accomplished by passing a continuous thin filament of thermoplastic material through a heated nozzle, which melts the thermoplastic material and applies it to the structure being printed. The heated material is applied to the existing structure in thin layers, melting and fusing with the existing material to produce a solid finished product.
The filament used in the aforementioned process is generally produced using an extruder. In some instances, the extruder may include a specially designed screw rotating inside of a barrel. The barrel may be heated. Thermoplastic material in the form of small pellets is introduced into one end of the rotating screw. Friction from the rotating screw, combined with heat from the barrel softens the plastic, which then is forced under pressure through a small opening in a die attached to the front of the extruder barrel. This extrudes a string of material which is cooled and coiled up for use in the 3D printer as the aforementioned filament of thermoplastic material.
Melting a thin filament of material in order to 3D print an item is a slow process, which is generally only suitable for producing relatively small items or limited number of items. As a result, the melted filament approach to 3D printing is too slow for the manufacture of large items or larger number of items. However, 3D printing using molten thermoplastic materials offers many benefits for the manufacture of large items or large numbers of items.
A common method of additive manufacturing, or 3D printing, generally includes forming and extruding a bead of flowable material (e.g., molten thermoplastic), applying the bead of material in a strata of layers to form a facsimile of an article, and machining such facsimile to produce an end product. Such a process is generally achieved by means of an extruder mounted on a computer numeric controlled (CNC) machine with controlled motion along at least the X, Y, and Z-axes. In some cases, the flowable material, such as, e.g., molten thermoplastic material, may be infused with a reinforcing material (e.g., strands of fiber) to enhance the material's strength. The flowable material, while generally hot and pliable, may be deposited upon a substrate (e.g., a mold), pressed down or otherwise flattened to some extent, and leveled to a consistent thickness, preferably by means of a tangentially compensated roller mechanism. The flattening process may aid in fusing a new layer of the flowable material to the previously deposited layer of the flowable material. In some instances, an oscillating plate may be used to flatten the bead of flowable material to a desired thickness, thus effecting fusion to the previously deposited layer of flowable material. The deposition process may be repeated so that each successive layer of flowable material is deposited upon an existing layer to build up and manufacture a desired component structure. When executed properly, the new layer of flowable material may be deposited at a temperature sufficient enough to allow a new layer of such material to melt and fuse with a previously deposited layer, thus producing a solid part.
In the practice of the aforementioned process, a major disadvantage has been encountered. Material extruders, of the type used in near net shape 3D printing, are designed to operate at a constant steady rate in order to produce a steady, consistent homogeneously melted plastic bead. In most cases, however, the majority of heat energy required to melt the plastic is generated by friction from a screw turning inside a barrel. This steady extrusion rate, however, creates difficulties when 3D printing. Specifically, the computer numeric controlled (CNC) machine used to move the extruder-based print head cannot start and stop instantaneously, and must, by necessity, vary in speed as it traces the path required to print the part.
This combination of a machine moving at variable speeds and an extrusion head outputting material at a constant rate results in a print bead that could vary in size. That is, the bead is thicker when the machine head is moving slowly, and thinner when the machine operates at a relatively higher speed.
A common approach employed in addressing the aforementioned problem is to servo-control the extrusion screw, speeding it up when the machine is moving faster and slowing it down as the machine motion slows. Since much of the energy used to melt the plastic is generated by rotation of the screw in the barrel of the extruder, varying the speed not only varies the rate by which material is pumped through the extruder but it also varies the amount of heat energy generated for melting the flowable material, such as, e.g., thermoplastic. The consequential increased temperature results in the thermoplastic material being less viscous; and, therefore, flowing faster than when it is cooler and thereby more viscous. The effect is that the flow rate from the extruder at any point in time is determined not only by the rotational speed of the extrusion screw, but also by the recent history of rotation, which determines how hot and thus how viscous the melted material is. This means that in a system where the rotation speed of an extruder varies randomly with time, the amount of material flowing from an extruder at a specific rotation speed will not be at a constant rate. Therefore, if the extruder screw is servo-controlled to operate at a specific rotational speed for a specific velocity of the print head, the resulting printed bead will not be consistent. Thus, method and apparatus are needed to produce a consistent print bead size when 3D printing.
Furthermore, the extruder may function to take polymer material in pellet form, heat, soften, and mix the material into a homogenized melt, and then pump the melt under pressure into a die to form the material into a useful extruded shape. This may be accomplished by providing an auger-type screw rotating inside a heated barrel, for example. The geometry, clearances, composition, and functionality of the screw and a barrel of the extruder may be determined as necessary to provide an extruder that operates as desired.
The extruder may be provided with the goal of completely mixing the melted material (e.g., polymer material) into a smooth, consistent form with no unmelted pellet portions or temperature variations in the melted material. One method of achieving this objective includes installing a breaker plate at the exit end of the extruder. A breaker plate may be, for example, a disk or plate that has a series of holes that provide resistance to the flow of the polymer melt. The holes in the breaker plate may be uniform holes approximately 0.125″ inches in diameter, and may be machined through the entire thickness of the breaker plate so as to be aligned with the flow direction of the polymer melt. This breaker plate may restrict the flow of material, increasing pressure inside the extruder barrel which assists in the melting and mixing process. One or more mesh screens or filters may be installed before the breaker plate to further restrict flow and increase pressure to aid mixing.
There may an optimal pressure range within which a particular extruder operates most effectively. Generally, a breaker plate and one or more mesh screens are installed in an effort to generate and maintain this desired pressure during operation of the extruder. While the inclusion of a breaker plate and/or screen may improve some mixing characteristics, they may also introduce drawbacks. For example, the additional restriction to flow may reduce throughput. Additionally, different polymers may require different breaker plate and/or screen configurations to achieve a desired pressure. Thus, the breaker plate and/or screen may need to changed each time the polymer being extruded changes in order to achieve a desired pressure that corresponds to the extruded polymer.
Another approach to achieve enhanced mixing may be to include knobs or other shapes on the extrusion screw, creating a “mixing section” which agitates the melt. This approach may also reduce flow of the melt and, in some cases, the friction caused by the mechanical mixing action can create unwanted heat in the mixing section.
Another purpose of the screen and/or breaker plate may be to create a generally fixed amount of resistance to the material flow in the extruder. This may facilitate generating and maintaining a steady state melt process within the extruder. If the breaker plate and/or screen were not in place in a typical extruder configuration, the amount of resistance to melt flow, and thus the operating pressure inside the extruder may depend solely or nearly primarily on the amount of resistance created by the shape of the forming die through which the melt flows after exiting the extruder. It may then become difficult to achieve consistent operation since the extruder may process material through a variety of different die shapes, each with a different resistance to flow. Some dies may generate insufficient resistance to flow to achieve optimal operating pressure while others may generate significantly higher pressure than is desired.